Not getting much love over in the People forum on this. My questions are really directed towards lighting so I probably should have posted here instead. Whether indoor or outdoor, how can I light a moving subject (even if it's just a person sitting still) at slow shutter speeds and prevent ghosting and motion blur?

If I were to bump up the power to get them 100% in "flash," the light on them would be much, much brighter than the background, right? I just couldn't figure it out. I didn't want to shoot at anything wider than f/5.6 or so to maintain a good DOF. The shots were in a dimly lit room and required a SS of <1/10.

This thread illustrates what I'm referring to, please respond in either forum.

Rear curtain will not help when you are shooting people with 0.8s exposures!

I assume that the reason you are using such low speeds is that you want to get a lot of the ambient light in. However you can do the same by using a faster shutter speed, with wider apertures and higher ISO.

For example instead of f/8 ISO 250 if you use f/4, ISO 800 your exposure time will go down to around 1/20s instead of 0.8 second. And if you are worried about noise then don't - with your main subject exposed by flash, ISO 800 on a 5DII will be essentially noiseless.

Rear curtain will not help when you are shooting people with 0.8s exposures!

I assume that the reason you are using such low speeds is that you want to get a lot of the ambient light in. However you can do the same by using a faster shutter speed, with wider apertures and higher ISO.

For example instead of f/8 ISO 250 if you use f/4, ISO 800 your exposure time will go down to around 1/20s instead of 0.8 second. And if you are worried about noise then don't - with your main subject exposed by flash, ISO 800 on a 5DII will be essentially noiseless....Show more →

no_surrender wrote:
And when I DO want to shoot at f/8 or smaller for adequate DOF?

Some of the shots were at 160-200mm...I was only about 12 feet away.

Then you have to rely on using higher ISOs. There is no way you are going to avoid motion blur with 0.8s exposures even if you use rear curtain. When using flash I have no problem using up to ISO 1600 if needed even with APS-C cameras. With a 5DII it should be OK to go up to ISO 3200 if needed. The noise will mainly be in the darker background parts which are exposed by ambient light only and those will typically be out of focus anyway. Plus a little bit of noise will be far less objectionable than the large motion blur that you will get otherwise. I wouldn't want to let your exposures go below 1/20s or so.

curious80 wrote:
Then you have to rely on using higher ISOs. There is no way you are going to avoid motion blur with 0.8s exposures even if you use rear curtain. When using flash I have no problem using up to ISO 1600 if needed even with APS-C cameras. With a 5DII it should be OK to go up to ISO 3200 if needed. The noise will mainly be in the darker background parts which are exposed by ambient light only and those will typically be out of focus anyway. Plus a little bit of noise will be far less objectionable than the large motion blur that you will get otherwise. I wouldn't want to let your exposures go below 1/20s or so....Show more →

Thanks, curious. Sure wish I had access to a location to practice these types of shots. I tried two weeks ago to get information and where this would take place, but didn't get in touch with the event POC until the day before only to be told I wouldn't be able to go in until a few hours prior. Feeling pretty discouraged right now. Next time I get a chance, I'll practice using a higher ISO. I was reluctant to try this because I was worried the the subjects would be blown out from the flash. Should have just lowered the power after cranking up the ISO...

Rear curtain won't prevent ghosting, it will just move it from one direction to the other. The only thing that will stop ghosting is having your flash/strobe power far enough above ambient so that the ambient doesn't record movement while the shutter is open. You can take a 10 second exposure without ghosting if the room is completely dark.

The rear curtain will at least freeze the subject on top of the blur so they don't appear to be disappearing. But yeah, I didn't check the exit last night, and with that info, the best way to fix this would be with higher iso, a slightly slower shutter, and a larger aperture.

If you shot these at iso 800, f5.6, 1/60th, and rear sync, you'd be golden.

I'm responding mostly to add my voice to the "rear-curtain does not prevent ghosting" choir. However, it's not as simple as "rear-curtain doesn't do that."

First, understand that all rear-curtain does is fire the flash immediately before the shutter closes, as opposed to normal flash that fires immediately after the shutter opens. The problem you're having is not with flash timing. The problem is with the shutter speed being too long.

There are two common uses for rear-curtain flash. The first is to make those cool images of cars where you have bright streaks from the lights, and then a nice sharp image of the car. Rear curtain is used to make sure the streaks are behind the car. With normal flash sync, the streaks end up in front of the car...which usually doesn't look good.

The second use of rear-curtain sync is to capture backgrounds. A typical example is of a riverside cityscape shot. Let's say you have a person standing by a river at night, and there's a nice cityscape across the river. You need two separate exposure for the shot...you need a flash exposure for your subject, and an ambient exposure for the cityscape. This is where rear-curtain sync works best. The shutter will open for an extended amount of time to capture the dim cityscape. Meanwhile, your subject is in the dark and standing still, waiting for the flash. Right before the shutter closes, the flash fires, illuminating your subject. You've now properly exposed both your subject and your background.

In the situation I described above, rear-curtain is used and no ghosting occurs. However, that's only because the subject was too dark to cause any ghosting. When both your subject and the background are being illuminated by the same light, the successful use of rear-curtain flash is dubious. Even when people are standing still, they're not standing still. No one can hold perfectly still for even half a second. In fact, your images show camera blur...even YOU can't hold steady at the shutter speeds you where using!

To start solving your problem, you first need to really learn about lighting. I suggest the book Light Science and Magic.

To address your specific problem you simply need to increase the shutter speed to suppress ambient light. Set your shutter speed to 1/120s, and allow your lighting to provide all the light for your scene. If you want the background illuminated, then get another light and put it on the background. Another possible solution is to illuminate the scene by bouncing light from the ceiling. But no matter what...you need to increase the shutter speed. In this situation, ambient light is your enemy and you need to get rid of it.

jared_irl wrote:
The rear curtain will at least freeze the subject on top of the blur so they don't appear to be disappearing.
Rear curtain doesn't do that either. There's no layering of light on the sensor...the blur is the same regardless of when the flash fires. The only difference is the location of subjects...caught either at the beginning of their movement with normal flash or at the end of their movement with rear-curtain flash.

pr4photos wrote:
You get a sharp subject with slow shutter and rear curtain flash...

No, if the subject is moving at all you'll have motion blur; the flash will only create a sharp image along with the blurred image, and the choice of first-curtain or second-curtain sync will only determine where the two images are in relation to each other.

How much blur depends on how much the subject moves, how much ambient light there is, and what shutter speed is used, not on which sync mode is used.

pr4photos wrote:
...but the subject won't be solid, you will be able to see through them to the background.

Only if the subject moves into position after the background exposure starts but before the flash fires; and the same would be true with front-curtain sync if the subject left the scene immediately after the flash.

no_surrender wrote:
...Whether indoor or outdoor, how can I light a moving subject (even if it's just a person sitting still) at slow shutter speeds and prevent ghosting and motion blur?

So I just looked at your photos from the other thread (most have been deleted, but there were still two that I can comment on), and subject movement doesn't appear to be an issue in those two. The subjects (a girl on chair in one, and a group of kids in the other) appear reasonably sharp in both, but the background -- particularly the lights on the tree -- is showing motion blur.

One thing would fix it in these examples even given the shutter speeds and apertures in use: a tripod. Your subjects weren't moving too much, but your camera was.

Remember the rule of thumb about shutter speed and lens length: 1/focal length = shutter speed. If you have a stabilized lens or camera you can slow the shutter by a few stops without camera shake being an issue, but only a few stops; any slower than that, and you need a camera support.

pr4photos wrote:
You get a sharp subject with slow shutter and rear curtain flash but the subject won't be solid, you will be able to see through them to the background.

This is not necessarily so and is in fact probably wrong in the majority of cases.

The only thing rear- vs. front-curtain sync changes is whether the flashed image is captured at the beginning of the exposure (and any motion taking place during that exposure) or at the end. The visibility of any motion blur depends on:
- the shutter speed
- how much the subject/subjects is/are moving
- how much the camera is moving
- how bright ambient-lit moving elements are in the image (including the brightness of those ambient-lit elements relative to the brightness of the flash-lit elements and the brightness of those ambient-lit elements relative to other ambient-lit elements)

If you are getting motion blur with front-curtain sync, you will still get it with rear-curtain sync assuming other variables remain the same.